For example, as shown in FIG. 18, in a conventional digital receiver 101, a tuner section 111 selects a signal of a desired channel from an RF (radio frequency) input, and converts the signal to an intermediate frequency signal having an intermediate frequency, and an intermediate frequency processing section 112 amplifies the intermediate frequency signal, then a demodulator 113 demodulates the intermediate frequency signal and takes out a baseband signal. Besides, the tuner section 111 and the intermediate frequency processing section 112 are provided with AGC (Automatic Gain Control) amplifiers 155 and 172, respectively, whose gains can be adjusted, and the demodulator 113 controls the gains of the AGC amplifiers 155 and 172, so as to keep input level to the demodulator 113 constant.
Here, in order to maintain excellent sensitivity and not to degrade demodulation performance such as signal distortion all over a dynamic range even when an RF input with a wide dynamic range is provided, the demodulator 113 estimates a signal level of the RF input based on the sum of the gains directed to the AGC amplifiers 155 and 172, and changes distribution methods for distributing a gain to the AGC amplifiers 155 and 172, according to whether or not the signal level of the RF input exceeds a predetermined TOP (Take Over Point).
Specifically, when the signal level of the RF input is less than the TOP, the demodulator 113 keeps the gain of the RF-AGC amplifier 155 at maximum and controls the gain of the IF (Intermediate Frequency)-AGC amplifier 172, so as to maintain the input level to the demodulator 113 constant. With this structure, the reduction in an S/N ratio can be restrained when the RF input is weak. On the contrary, when the signal level of the RF input exceeds the TOP, the demodulator 113 keeps the gain of the IF-AGC amplifier 172 constant and controls the gain of the RF-AGC amplifier 155, so as to maintain the input level to the demodulator 113 constant. This structure can prevent a malfunction that, as a result of an input of an excessive-level signal to a circuit located later than the RF-AGC amplifier 155, waveform distortion is caused and demodulation performance is degraded. By changing the foregoing distribution methods, the digital receiver 101 can achieve high receiving sensitivity and low waveform distortion simultaneously all over a wide input dynamic range.
However, in the foregoing conventional structure, since the demodulator 113 provided later than the tuner section 111 and the intermediate frequency processing section 112 estimates the signal level of the RF input based on the sum of the gains directed to the AGC amplifiers 155 and 172, and changes the distribution methods for distributing the gain to the AGC amplifiers 155 and 172, according to whether or not the signal level of the RF input exceeds the predetermined TOP (Take Over Point), when there is manufacturing dispersion in a member constituting the tuner section 111 or the intermediate frequency processing section 112, the TOP deviates from an optimum value, making it difficult to achieve both high receiving sensitivity and low waveform distortion.
Here, if the TOP is set high to obtain high receiving sensitivity, waveform distortion is likely to occur, and if the TOP is set low to restrain waveform distortion, receiving sensitivity is declined. Thus, since high receiving sensitivity and low waveform distortion have trade-off relationship, it is necessary to set the TOP very critically so as to achieve the both in high levels.
Meanwhile, in the tuner section 111, since a high frequency signal is processed, the range of a gain deviation is relatively large, and it is extremely difficult to restrain the gain deviation all over a receiving band. Further, in the tuner section 111, it is difficult to adjust tracking completely, resulting in a large frequency deviation in a gain. Depending on channels, there are some cases where gain dispersion not less than 10 dB is caused.